Various padding devices have been employed in the past. Examples include liquid- or gas-filled bladders, e.g. water-filled cushions and pneumatic pads; and gases or liquids dispersed in a solid material, e.g. foams and gels. Generally, such padding devices operate on the principle of conformation to the shape of an object when placed under pressure. When a force, such as a person's mass, is placed on such a padding device, the device deforms so as to conform to the shape of the pressure applying object in order to distribute the force over as large an area as possible. These devices perform adequately when the object being padded has a relatively large, uniformly shaped surface area. However, when the object being padded includes a relatively small area of concentrated force, such as that caused by a sharp protuberance, the prior art padding devices often do not perform adequately. This is because the padding devices exert greater pressure on the area of concentrated force.
The reason for the greater pressure is that the prior art padding devices deform by compressing. Therefore, a sharp protuberance would compress the padding device more than the surrounding area and, as a result, the padding device presses back with greater force in these areas. Such areas of high pressure are especially undesirable when the protuberance is a bone, such as an ankle or ischial tuberosity. The high pressure can lead to discomfort and, after periods of extended use, actual damage to the tissue overlying the protruding bone.
The problem can be described with reference to a padding device comprising a gas dispersed in a solid material, e.g. foam. Tiny gas bubbles in foam act like millions of coil "springs". When required to conform to an irregular shape, such as a human body, the "springs" are compressed to varying degrees, each pushing back on the body with a force proportional to the amount of compression. Intimate conformity is best obtained with a relatively soft foam, which can be compared to weak "springs". The pressure on protuberances, where the "springs" are greatly compressed, will be relatively high, possibly causing pain and reduced circulation. The problem is even more pronounced if a stiffer foam is employed and the "springs" are stronger.
In order to alleviate the problem of differential pressure inherent with many prior art padding devices, a flowable, pressure-compensating material was developed. This material is described in U.S. Pat. No. 3,237,319 by Alden Hanson, issued Mar. 1, 1966; U.S. Pat. No. 3,402,411 by Alden Hanson, issued Sept. 24, 1968; U.S. Pat. No. 3,635,849 by Alden Hanson, issued Jan. 18, 1972; U.S. Pat. No. 4,038,762 by Swan, Jr., issued Aug. 2, 1977; U.S. Pat. No. 4,083,127 by Chris Hanson, issued Apr. 11, 1978; U.S. Pat. No. 4,108,928 by Swan, Jr., issued Aug. 22, 1978; U.S. Pat. No. 4,144,658 by Swan, Jr., issued Mar. 20, 1979; U.S. Pat. No. 4,229,546 by Swan, Jr., issued Oct. 21, 1980; and U.S. Pat. No. 4,243,754 by Swan, Jr., issued Jan. 6, 1981. Each of these U.S. patents is hereby incorporated by reference in its entirety.
In use, the flowable, pressure-compensating materials disclosed in the above-mentioned patents are typically placed in a pliable package, such as between two leak-proof resinous sheets which are sealed at the edges. The flowable materials act hydraulically. An applied force causes flowable material to migrate from areas of higher pressure to areas of lower pressure until pressure throughout the package is uniform. Once conformity has been achieved, force is distributed substantially equally over the entire surface of the package thus alleviating the differential pressure problems associated with prior devices. The viscosity of the flowable material can be varied. Higher viscosity does not decrease the ability of the flowable material to conform to the shape of the pressure applying object, only the speed at which it will migrate to conform. Flowable materials are presently sold under the registered trademark FLOLITE (TM).
In spite of the advantages provided by the flowable materials, there are certain applications where improvement is desirable. In particular, improvement in the control of migration would be advantageous. Flowable material is presently typically packaged in a leak-proof container comprising two resinous sheets heat sealed together. One problem which occurs with this type of packaging is "bottoming out". Bottoming out occurs when the layer of flowable material in the package is insufficient to fully surround and support an object. As a result, a protuberance on an object can cause all of the flowable material to migrate laterally until no flowable material remains under the protuberance, and consequently the protuberance rests on the surface below the flowable material package. This reduces the effectiveness of the padding and increases the pressure on the end of the protuberance.
In order to alleviate the problem of excessive migration, additional flowable material can be added to the package. But this solution is unsatisfactory when the weight of the total package must be held to a minimum. Another possible solution is to restrict the migration of the flowable material by decreasing the effective surface area by segmenting the package into individual compartments. However, this deleteriously restricts the ability of the flowable material to conform to various shapes.
One application where the provision of adequate padding is problematic is on bicycle seats. Bicycle seats must be of relatively small size so that the seat itself does not interfere with the pedaling action of the rider's legs. Therefore, the rider's weight is concentrated over a relatively small area. This weight is further concentrated on the rider's ischial tuberosities. Additionally, the weight of the seat should preferably be held to a minimum. Several patents have issued for bicycle seats and bicycle seat covers which were designed to overcome the padding problems inherent in bicycle seat design.
U.S. Pat. No. 4,504,089 by Calvert et al., issued Mar. 12, 1985, discloses a liquid cushioned bicycle seat having separate forward and rear compartments. The forward pommel compartment has a longitudinal baffle to restrict side-to-side flow of the cushioning liquid. The rear saddle compartment has two side sections separated by a barrier which tapers towards the rear. The barrier terminates before reaching the rearward end of the saddle, thereby providing an opening connecting the side sections which permits side-to-side flow of the cushioning liquid. The pressure exerted on the first and second side sections cyclically increases and decreases within each section as the bicycle is pedaled. The rear opening serves as a baffle to control the direction and movement of cushioning liquid from one section to the other upon compression caused by pedaling.
The liquid cushioning media disclosed by Calvert et al. is preferably water. Due to water's low viscosity, it conforms to the shape of a pressure applying object almost instantaneously. Logically, it would appear that water would be an excellent cushioning material because of its capability to instantaneously flow to achieve conformability. However, incompressible materials which provide almost instantaneous conformability do not provide as much control and stability as padding materials which do not respond to instantaneously applied pressure. Anyone who has attempted to sit on a waterbed is familiar with the lack of stability associated with water cushioned devices.
U.S. Pat. No. 3,807,793 by Jacobs, issued Apr. 30, 1974, and U.S. Pat. No. 3,997,214 by Jacobs, issued Dec. 14, 1976, disclose compartmentalized bicycle seat covers. The compartments can be filled foam rubber or a thixotropic dilatant material. The thixotropic or dilatant material is defined as having a yield point of at least 4 grams per square centimeter and a viscosity between about 100,000 poise and about 40,000 poise at 0.degree. F. and 80.degree. F. This definition describes materials which are includable in the group of flowable, pressure-compensating materials described hereinbefore. The thickness of the padding is kept to a minimum and it is designed to conform to the pelvic configuration of the user and to retain that shape. In one embodiment, the padding material is a pressurized foam rubber which is injected in the compartments while the user is seated on the bike seat. The foam rubber material is claimed to have a viscosity substantially lower than the thixotropic or dilatant material. The cover can be clipped to a bike seat by clipping the rear end and using an elastic to secure the front end.
While the bicycle seat covers disclosed in the two Jacobs patents include the use of flowable materials in a compartmentalized manner, the designs disclosed therein suffer from a number of disadvantages. For example, while the compartmentalized containment of the flowable material in relatively thin layers of small area can help reduce bottoming out of sharp protuberances, it also limits the conformability of the flowable material over broader areas. In other words, as the area of the flowable material compartment decreases, the ability to conform to various shapes is reduced. Another disadvantage of the Jacobs design is the inability of the flowable material in the rear portion of the seat to flow from one side of the seat to the other. This prevents the seat from adjusting to anatomical differences between different riders, or asymetries of a single rider, e.g. a rider who may sit with one pelvic bone closer to the seat than the other. Another disadvantage of the Jacobs design is that the cushioning material is capable of traveling down the center portion of the seat. As a result, if a rider rides forward on the seat, the pressure applied to the forward portion forces cushioning material towards the rear central portion. As can be appreciated, an excess of cushioning material in this area of the seat would be uncomfortable over periods of even short duration.
U.S. Pat. No. 3,844,611 by Young, issued Oct. 29, 1974, discloses a bicycle seat having two layers of resilient pad, a first upper layer of relatively low density and a second lower layer of relatively high density. The pad can comprise any resilient material, for example, sponge rubber, polyurethane and vinyl foams or the like. U.S. Pat. No. 4,429,915 by Flager, issued Feb. 7, 1984, discloses a bicycle seat having adjustable foam inserts to customize the seat to the rider. The inserts are formed from a foam such as polyurethane, preferably in a variety of densities or hardnesses. An air-filled tube is also used to adjust the shape of the seat. Different areas of the seat have different degrees of hardness or softness. U.S. Pat. No. 4,733,907 by Fellenbaum, issued Mar. 29, 1988, discloses a bicycle seat comprising foam having a density of 1.8-2.8 pounds per cubic foot and an impression force deflection of between 75 and 95 IFD. The foam is said to be resilient and preferably of an open cell nature, because a closed cell foam would be too stiff. The foam is preferably 1-1.5 inches thick. The foam over the front tongue can be as thin as 1/4 inch.
One problem encountered with padding devices for bicycle seats, such as those disclosed in the patents by Young, Flager, and Fellenbaum, is that the resilient foam and/or air-filled tube disclosed in these patents provide substantially instantaneous conformability to the rider's shape. This is consistent with the long-held belief that a resilient, and therefore nearly instantaneously conforming, cushioning or padding device provides the best padding for a seat such as a bicycle seat. However, such padding devices do not evenly distribute weight, and the resulting pressure differential is undesirable in certain applications.
In summary, prior art padding devices which incorporated resilient materials or air- or water-filled tubes have been found to perform inadequately in certain applications. Additionally, bicycle seat padding devices which use flowable materials, such as those disclosed by Jacobs, have been found to have design deficiencies. Specifically, the compartmentalized design prevents total conformability and side-to-side movement of the flowable material, while permitting undesirable forward to rearward movement of the flowable material down the center portion of the bicycle seat. Therefore, it would be advantageous to have a padding device which overcame the deficiencies of the prior art.